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Nanofiltration of treated digested agricultural wastewater for recovery

of carboxylic acids

Myrto-Panagiota Zacharof*a,b,c

, Stephen J. Mandale a,b,c

, Paul M.Williams a,b,c

and Robert W. Lovitt a,b,c

a Centre for Complex Fluid Processing (CCFP), College of Engineering, Swansea University,

Talbot building, Swansea, SA2 8PP, UK

b Centre for Water Advanced Technologies and Environmental Research (CWATER),

College of Engineering, Talbot building, Swansea University, Swansea, SA2 8PP, UK

c Systems and Process Engineering Centre (SPEC), College of Engineering, Swansea

University, SA2 8PP, UK

Abstract

Synthetic solutions of varying concentrations of carboxylic acids, namely acetic and butyric

acids (50 mM, 100 mM) and treated digested agricultural wastewater with a carboxylic acids

concentration of 21.08 mM of acetic acid and 15.81 mM of butyric acid were processed with

a range of nanofiltration membranes and enrichment schemes to concentrate carboxylic acids.

The study was conducted with the scope of platform chemicals recovery from complex

effluents, investigating the feasibility of nanofiltration as a method of choice. Membrane

filtration is easily scalable into various arrangements, allowing versatility in operation and

enrichment treatments, which other recovery practices such as liquid extraction do not allow.

Among the five nanofiltration membranes used (NF270, (Dow Chemicals, USA), HL, DL,

DK, (Osmonics , USA), LF10 (Nitto Denko, Japan)) the DK, DL and NF270 were identified

as the best candidates for carboxylic acids separation and concentration from these complex

effluents, both in terms of retention and permeate flux. These membranes achieved retention

ratios, up to 75% giving retentates up to 53.94 mM acetate and 28.38 mM butyrate for the

agricultural wastewater. Effluents were modified by the addition of alkali and salts (sodium

chloride and sodium bicarbonate), and it was found that highest productivity, retention and

flux was achieved at pH 7 but at higher pH there was a significant reduction in flux.

Keywords: acetic acid; butyric acid; effluents; nanofiltration; retention; wastewater

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Graphical Abstract

Highlights

Carboxylic acids retention relied on membrane type and environmental conditions.

Enhanced retention of acids at pH 8.5 of all 5 nanofiltration membranes tested.

CaCl2 and NaHCO3 additions gave further increase of acid retention by membranes.

Acid retention was better with treated wastewater than simple synthetic solutions.

Improvements in acid retention are possible by appropriate solution manipulation.

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1. Introduction

The decarbonisation of energy generation is recognized as a high priority among European

countries and the United States (DECC, 2014). Throughout the European continent but also

in the United States, the carbon based economy is challenged due to fossil fuel scarcity

making the generation of electricity using fuels from renewable sources an attractive option

(www.biogas-info.co.uk). The European Union has adopted a framework of directives to

uncouple the energy reliance on fossil based fuels. The United Kingdom has been entrusted

to achieve 20% of its energy consumption from renewable sources by the year 2020, stating

the emergent character of the situation (Bauer et al., 2009). This will result in a reduction of

greenhouse emissions, decelerating climate change as well as partially securing energy

efficiency.

Among the numerous technologies such as tidal, solar and wind energy, proposed to achieve

this goal, biogas for combined heat and electricity production through anaerobic digestion has

been acknowledged to be effective and suited to the countrys agroindustrial economy,

resulting in the development of over 100 sites (www.biogas-info.co.uk). Being of relatively

simple construction, enhancing local and national economies through supporting small and

medium sized companies and relying on a well know and widely investigated process of

anaerobic fermentation of materials including food, feed and plant origin, anaerobic digesters

have spread rapidly throughout the country (www.biogas-info.co.uk). Present uses of

anaerobic digestion are focussed on production of electricity and stabilization of domestic

and municipal sludge and wastewater carried out at large wastewater treatment plants. The

main advantages of this process are a reduction in the volume of waste sludge with methane

gas production, while the simultaneous release of ammonia due to organic matter hydrolysis

should be carefully taken into consideration (Bauer at al. 2009).

When the process comes to an end, the remaining highly viscous fluids, rich in ammonia,

phosphate, acids, and metals are a significantly growing problem. Their disposal untreated,

by land spreading might be hazardous, contaminating soil, ground and surface water, causing

eutrophication and imposing significant environmental hazards (Jung and Lovitt, 2011;

Tyagi and Leo, 2013). There are also human health concerns due to land related

pathogenicity contained in the raw materials (Jung and Lovitt, 2011; Tyagi and Leo, 2013).

These concerns have highlighted the problems of sludge disposal and the necessity of

solutions addressing them.

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Regardless of their environmental impact these effluents can have a considerable market

value, due to their rich content in valuable nutrients, that if recovered will contribute

significantly to the sustainability of the low carbon economy developed throughout the

country. For example, platform chemicals that are currently produced from petroleum, like

acetic acid have a market size of 3,500,000 t/yr with a market value of 800 USD/t while

butyric acid reaches 2000 USD/t and a market size of 30,000 t/yr (Zacharof and Lovitt,

2013).

Conventional treatment of liquid waste is becoming increasingly expensive, demanding

significant amounts of effort, resources and energy to be safely discharged into the

environment (Zacharof and Lovitt, 2013; Zacharof and Lovitt, 2014a). Currently living in a

low carbon, knowledge driven economy, with growing awareness over environmental

protection due to climate change and natural resources exhaustion; the need to recycle, reuse

and recover energy and valuable chemicals from wastewater becomes apparent (Kertest and

King 1986; Dimakis et al., 2011; Tyagi and Leo, 2013). Waste can be seen as a virtually

inexhaustible resource, being utilized in industrial markets to generate combined heat and

power, fertilizers, chemicals, feeds and food in the developed world (Jefferson, 2008). Within

the following decade, driven largely by legislative, environmental, economic and social

drivers, these markets will be further developed. They will be shifting into recovering

chemicals from the waste such as ammonia, phosphate, carboxylic acids (CA) and metals in

an effort to reduce the carbon footprint of their production and limit their manufacture by

utilizing natural resources achieving environmental sustainability. These activities will be

constituting waste safe for environmental discharge in the form off particle, nutrient free and

sterile effluents. Therefore the utilization of waste as a valuable commodity and platform

chemicals mine is an important step to the development and deployment of alternative

sources for energy production (Tyagi and Leo, 2013).

Membrane technology is a rapidly developing easily scalable technology with numerous

arrangements and alternatives, often easy to incorporate and integrate into waste treatment

processes. They offer high productivity and low operational cost compared to other

competing technologies, since there is no phase change of water and minimal or no use of

chemical additives (Cho et al., 2012, Zacharof & Lovitt, 2014 a). The use of membranes in

the industry as a downstream and upstream processing option has been proven to be an

attractive, cost effective option, applicable in numerous well defined waste systems (Tahri et

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al, 2012). Examples in the food industry include, processing waste of alcoholic (beer) and

non alcoholic beverages (orange juice), edible oils (olive, vegetable) (El-Abbassi et al.,

2014), vegetative proteins (soy) (Cassini et al., 2010) and dairy (whey, milk), while

examples from the chemical industry include processing of tanning wastewater (Kim, Park &

Kim, 2005) , electroplating effluents as well as effluents rich in hazardous chemicals (Al-

Amoudi, 2010).

Of all the pressure-driven membrane processes, nanofiltration is the most prominent

candidate for the recovery of CA due to their negative charge and low molecular weight,

since it employs various mechanisms including steric based exclusions (namely size or

molecular weight), shape and charge. Nanofiltration has been applied successfully (Koyuncu

2002; Kimura et al., 2003, Zhou et al., 2013) for the removal of hazardous materials, such as

metals or endocrine disruptors of waste effluents as well as for the recovery of low molecular

weight chemicals including enzymes and proteins.

Therefore, this work reports on the nanofiltration of carboxylic acid mixture solutions of

known concentrations of acetic and butyric acid and treated agricultural anaerobically

digested wastewater within the scope of acid recovery. The tested wastewater was pretreated

and then formulated to a sterile; large particle free solution using microfiltration. Five

commercially available membranes, tested with characterizing solutions, were used in a

bench scale arrangement. The filterability of the streams has been evaluated in terms of flux

using various treatment schemes, developed to enhance retention of the acids. The effluents

including the carboxylic acid solutions of known concentration and the treated wastewater

were modified by the addition of salts, sodium carbonate (Na2CO3), sodium bicarbonate

(NaHCO3), sodium chloride (NaCl) and calcium chloride (CaCl2) and alteration of pH using

sodium hydroxide (NaOH) or hydrochloric acid (HCl) in a range of 4 to 9 in an effort to

enhance their retention.

2. Materials and Methods

2.1. Materials

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Synthetic mixture solutions of known concentrations of acetic and butyric acid in a

concentration of 50 mM and 100 mM were prepared using glacial acetic acid (>99.7%) and

butyric acid (>99%) provided by Sigma-Aldrich, Dorset , United Kingdom in deionised

water.

Waste effluent streams (agricultural wastewater derived from spent agricultural digested

sludge namely a mixed waste of cattle slurry, vegetable waste and silage), taken off the

output line of the anaerobic digester, used for manure production, but before passing through

the automatic coarse particle separator (>5 mm), were collected from Farm Renewable

Environmental Energy Limited (Fre), Wrexham, United Kingdom (http://www.fre-

energy.co.uk). These samples were pre-treated using dilution, mixing, sedimentation and

sieving (Zacharof and Lovitt, 2014b). The resulting effluents were microfiltered through a

pilot scale unit equipped with a ceramic membrane (pore size

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stainless steel bench scale unit (HP4750) provided by Sterlitech, Kent, WA, USA. The

experimental set-up is shown in Fig.1. The unit was operated batchwise, being continuously

pressurised at 10 bar during filtration using nitrogen gas at a stirrer speed of 300 rpm. The

permeate was collected in a graduated vessel (100 mL) placed on an electronic two decimal

place precision scale (Sartorius PLS 303, UK). The balance was connected to portable

computer equipped with logging software, that enabled the user to obtain the permeate flow

rate as a function of filtration time. The cell unit, with a maximum process volume of 200

mL, was equipped with a magnetic stirrer and a membrane filter of an effective area of 14.6

cm2. A virgin membrane sample was used for each trial minimising fouling concerns. The

samples were filtered to give 10 mL retentate and 90 mL permeate.

2.2.3. Membrane Characterisation

Membrane characterisation studies using pH adjusted deionised water (pH 4 to 9 in steps of

1.5 pH units) were carried out to determine the influence of operating conditions on

permeate flux during filtration. The permeability of the characterising solutions was

measured in order to analyze the behaviour of the system (see Table 3).

2.2.4. Enrichment Schemes

The processing of the solutions within the scope of enhanced reclamation of carboxylic acids

was carried out using two schemes (see Fig.2): addition of various salt solutions in different

concentrations and pH adjustment in a range from 4 to 9. Four different salt solutions of

NaCl, CaCl2, NaHCO3 and Na2CO3 were prepared in 50 mM and 100 mM concentrations and

were added in a 1:1 ratio to the corresponding concentrations of the synthetic mixtures, and to

the treated agricultural wastewater. The pH was adjusted in steps of 1.5 pH units, in a range

between pH 4 and 9. This was achieved by drop wise addition 1M NaOH or HCl as

necessary, to the treated effluent and synthetic solutions.

2.2.5. Analysis of physicochemical characteristics and solids content of treated

digested agricultural wastewater

Parameters relating to solids content in the treated stream such as total solids (TS, g/L), total

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suspended solids (TSS, g/L), total dissolved solids (TDS, mg/L), alkalinity, and optical

density were determined according to Clesceri, et al. (1998). Laser diffraction was used to

determine the particle size distribution (PSD) of the treated effluent samples using a

Mastersizer 2000 (Malvern, UK) while a Zetasizer (Malvern, UK) was used to determine the

zeta potential. Conductivity and salinity of the samples were measured using a conductivity

meter (Russell systems, UK) calibrated with a standard solution of 0.1M KCl. Butyric and

acetic acid were determined using head space gas chromatography (Zacharof and Lovitt,

2014a). Each parameter was triplicated to obtain the average data (a standard deviation of

mean

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=M

CTR (3)

where R is the universal gas constant [(kJ/(kmol K)], T is the absolute temperature in Kelvin

(K) , C is the concentration of the solute (kg/m3) and M is the molar mass (kg/kmol) (Van

den Berg and Smolders,1990).

3. Results and Discussion

3.1. Characterization Study of the Nanofiltration Membranes

The permeability of the characterizing solutions through the membrane was measured to

analyze the behavior of the unit. The flux values for all the membranes decreased

simultaneously with the increase of pH, from acidic (pH 4) to alkali (pH 9) (Table 3).

It can be observed that at pH 4 the flux is significantly higher a phenomenon that can be

explained by the neutralizing of the membranes surface charge, as at acidic pH the surface

charge is approaching its isoelectric point (Al-Amoudi et al., 2007; Al-Amoudi & Lovitt,

2007). Previous research (Nystrom et al., 2004; Manttari and Nystrom, 2006; Manttari et al.,

2006;) on the membranes used in this study has shown that they are hydrophobic, as

suggested by contact angle measurements ranging between 45.1 (HL) to 56.7 (DK). It has

been previously indicated that at alkaline pH (pH 7 and above) there is a significant change in

the membrane surface charge, enhancing negative charges (Al-Amoudi et al., 2008); this

influences the flux by reducing the rate, a phenomenon that is observed with all the five

membranes. Among the five membranes, highest productivity in terms of flux is observed

with HL and DL membranes, while the smallest flux is given by LF10 followed by NF270

and DK.

3.2. Nanofiltration of Carboxylic Acids Synthetic Mixtures Using Enrichment

Schemes

Model solutions of acetic and butyric acids of 50 mM and 100 mM respectively were

prepared and filtered through the high pressure unit at 10 bar. Two enrichment schemes were

applied for their treatment, namely, pH adjustment and salts addition.

3.2.1. Nanofiltration of Carboxylic Acid Solutions Using pH Adjustment

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Numerous studies (Masse et al., 2007, Weng et al., 2009, Masse et al., 2010) have been done

regarding the nanofiltration of synthetic or model solutions of acids as a potential method for

reclamation of these acids; however, these have been mainly focused on a single acid

solutions rather than mixtures. Nanofiltration has not been the traditional method of choice

for recovery or purification of carboxylic acids, since they are normally petrochemical

derivatives. However, the rising cost of energy (Zacharof and Lovitt, 2013, Tyagi and Leo,

2013) and the gradual depletion of oil reserves has led to exploration of alternative options

such as the use of waste streams as sources (Volchek et al., 2002; Al-Amoudi and Lovitt,

2007; Al-Amoudi, 2010) calling for innovative cost effective methods of recycling. These

methods should be able to separate efficiently the acids from the multicomponent waste

streams, to provide solutions suitable for further economic waste free purification.

When a 50 mM binary mixture of acetic-butyric acid was filtered in a pH range of 4 to 9, the

highest flux occurred at pH 4, for all the five membranes, with HL offering the highest rate

and LF10 the lowest (Table 4). At pH 9, the flux is significantly reduced for all the

membranes, with smallest flux given by NF270 and higher by HL. Similar results have been

achieved in past research (Han and Cheryan, 1995) regarding a single component solution of

acetic acid, suggesting that there is a strong influence of pH on the flux of carboxylic acids.

A reduction in flux can be potentially correlated with an enhanced retention for both acids in

this dead end filtration system at pH 7 and above. Regarding the retention of carboxylic acids

in this system, it was found that acetic acid was better retained at pH 8.5 and 9 while at pH 4

the retention was considerably limited (Fig.3a). For butyric acid similar trends have been

identified, however retention is lower than acetic acid, with the exception of NF270 which at

pH 9 offers a higher retention rate than acetic acid (Fig. 3b).

To explore the influence of concentration on the components of interest in the feed and to

investigate whether it plays an important role on retention, the experiments were repeated

with a synthetic mixture at 100 mM. The flux for all the membranes is higher (Table 4) when

compared with the flux achieved under the same conditions with a 50mM solution. A similar

trend of reduced flux in alkali conditions (pH 7

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above (Fig.4b). On the other hand, at pH 4 varying trends in retention among the membranes

have been identified still, though, retention is very low for butyric acid compared to alkali

conditions.

Every membrane has a significantly different behavior, compared to the others when

separating and concentrating the acids, regardless of their similarities in material fabrication,

hydrophobicity and MWCO (Table 2). Hydrophobic membranes are preferred due to their

elevated negative charges in alkaline pH, a property that if judiciously manipulated, allows

the enhanced retention of carboxylic acids. It can also be seen that in the synthetic solutions

for both concentrations (Fig 3 a, b, Fig. 4 a, b), the retention of butyric acid is favoured over

the retention of acetic acid, a phenomenon that might be attributed to the higher molecular

weight (Van der Bruggen et al., 1999; Van der Bruggen et al., 2002; Van der Bruggen and

Vandecasteele, 2003). This highlights the potential of nanofiltration to be used as a selectivity

method enhancing the separation of substances of interest from complex solutions.

3.2.2. Nanofiltration of Carboxylic Acids Synthetic Mixtures Using Salts

Solutions

Previous research (Bellona et al., 2004; Choi et al., 2008; Umpuch et al., 2010) has shown the

effectiveness of salts for enhancing retention during nanofiltration of acids. Having identified

that treated agricultural wastewater is of high salinity, alkalinity and TDS, implying an

elevated content of salts, the effect of four different salts solutions namely, sodium carbonate,

sodium bicarbonate, sodium chloride and calcium chloride in two concentrations was tested.

When 50mM of sodium carbonate and bicarbonate was added to a 50 mM synthetic mixture

of butyrate and acetate, retention was improved for both acids, when compared to the

untreated feed (see Fig. 5 a, b). Sodium bicarbonate has a stronger effect than sodium

carbonate, while both agents favor the retention of acetic acid over butyric acid. Retention of

acetic acid without any treatment is low (Fig. 5a), while when sodium carbonate is added the

retention is at improved by 35.1% over the untreated value and sodium bicarbonate enhances

the retention by 187.9%. This result is repeated with butyric acid (Fig. 5b) where the

retention without any treatment is low (Rret = 30%) but this value is considerably improved

when adding sodium carbonate, by a factor of 51.4% while when sodium bicarbonate is

added the this value increases by 124.5% .

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The experimental process was repeated using the 100 mM synthetic mixture with 100 mM of

sodium carbonate and bicarbonate. A higher concentration of acids does not strongly

influence the retention, while similar trends are noticed. Sodium bicarbonate does on average

enhance the retention of acetic acid more than sodium carbonate, when compared to untreated

solutions (an increase by a factor of 78.6% versus 59.5% respectively). On the other hand for

butyric acid, retention was again improved by sodium bicarbonate by a 126.8%, when

compared with the untreated solution while sodium carbonate improves the retention by

85.2%.

Although the results are encouraging, suggesting that the addition of sodium bicarbonate

could indeed increase the retention of the acids and enable selectivity among the substances

of interest, the retention achieved is not exceptionally high. Further investigation using

sodium chloride and calcium chloride was performed (see Fig. 6 a, b). Both acetic and butyric

acid retention was improved, however, butyric acid retention was to a lesser extent, with

sodium chloride having a stronger influence than calcium chloride. In both concentrations,

sodium chloride enhanced the retention, by a value of 180.9% for the 50 mM acetic acid

solution and 162.7% for the 100 mM solution. On the other hand, the 50 mM calcium

chloride addition provides an 86.7% increase in retention and whilst the 100 mM solution

gives a 63.6% increase in retention for acetic acid (see Fig. 6a). A similar trend was found for

butyric acid (see Fig. 6b) with 50 mM of sodium chloride offering an average enhancement in

retention of 116.1% and a 100 mM an enhancement of 208.9%, while for calcium chloride

the enhancement is 67.9% and 100.4% for the 50 mM and 100 mM solutions respectively.

There is variation in the flux for each membrane on the addition of salts into the synthetic

mixtures, with the highest flux occurring with the addition of calcium chloride and sodium

carbonate, correlating with the minimized retention of the acids from the membranes. The

lowest flux was found for the LF10 membrane and the highest with the HL; while the NF270,

DL and DK lie in between (Table 5). It was expected that the addition of salts would further

contribute to the acids retention due to the raising of the osmotic pressure. The vant Hoff

equation has been used, although simple, can offer a first approximation of osmotic pressure,

based on the concentration of salts solutions. When sodium carbonate (50 and 100 mM) was

added the osmotic pressure is calculated to be 0.011 kPa and 0.023 kPa, for sodium

bicarbonate is 0.014 kPa and 0.029 kPa respectively, sodium chloride was 0.010 kPa and

0.021 kPa and calcium chloride was 0.005 kPa and 0.11 kPa. The osmotic pressure caused by

the addition of salts is found to be insignificant compared to the applied pressure and

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therefore not restricting the flux. The concentration polarization phenomenon might influence

the separation function of the membranes; however the low quantity of salts is potentially

contributing to a relatively small effect by this phenomenon, at this instance.

Both treatments do provide an optimized retention of the acids, with a pH treatment of 7 and

above achieving up to 65% of retention, similarly the addition of sodium carbonate and

calcium chloride to the synthetic mixtures. When sodium bicarbonate and sodium chloride

are added retention is improved drastically, reaching 87.3%, suggesting that these salts can

successfully be applied in the nanofiltration of carboxylic acid in order to concentrate and

separate carboxylic acids from mixtures.

3.3. Nanofiltration of Treated Digested Agricultural Wastewater Using

Enrichment Schemes

The initial trials on model solutions of acetic and butyric acid having shown promising results

regarding the retention of acids by membranes were then repeated using treated agricultural

wastewater. The results were expected to differ since the treated effluent is a multicomponent

solution (Table 1) containing particulates mainly composed of minerals that can potentially

enhance the retention of the acids.

3.3.1. Nanofiltration of Treated Agricultural Wastewater Using pH Adjustment

Previous research (Masse et al., 2008) has been focused on the removal of carboxylic acids

existing in anaerobically digested waste of various sources; however these were identified as

pollutants rather than a useful resource. pH alteration has been identified as an effective

treatment, previously tested on the model solution but also in the literature (Zacharof and

Lovitt, 2013).

Treated agricultural wastewater was filtered in the pH range of 4 to 9. The highest flux was

found at pH 4 for all five membranes, with HL offering the highest rate and LF10 the lowest

rate of flux (Table 6). At pH 9, the flux is significantly reduced for all the membranes, with

smallest flux given by the LF10 membrane and the higher flux by the HL membrane. When

compared to the results achieved with the model solutions, a similar trend was found,

although the highest flux throughout the whole spectrum was seen for the model solutions of

both concentrations.

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Enhanced retention has been found for both acids (Fig.7 a, b) at pH 7 and above, while at pH

4 the retention is considerably limited. For butyric acid similar trends have been identified,

but for pH with the DL membrane the phenomenon of negative retention was found at pH 4

and 5.5 (-12.80% and -14.70% respectively). These results have been confirmed by multiple

repetitions of the trials.

Negative retention is a common phenomenon in nanofiltration, previously observed when

filtrating complex solutions containing ions and substances of different charges such as

brackish water (Mnttri et al., 2006). Commonly this phenomenon occurs, when a strongly

negatively charged solute is better repelled from a negatively charged membrane; for

example divalent ions will be better retained than monovalent ions.

In these systems a divalent ion would be better rejected than a monovalent ion, a highly

negatively charged solute would be better rejected from a negatively charged membrane

(Hilal et al., 2008; Mandale and Jones, 2008; Kimura et al., 2009). The transport of the

negatively rejected compound is enhanced by the larger charged compounds on the retentate.

Conceivably, this phenomenon is governing this system. Acetic acid having a lower pKa

therefore is more easily disassociated and consequently ionised when compared to butyric

acid. This contributes to acetic acid being better retained, since the negatively charged

membrane surface is strongly rejecting the negatively charged molecules, enhancing the

amount of electrostatic repulsions between the membrane surface and the solute of interest.

The behavior of each membrane in this system varies significantly, with high retention results

being observed with the NF270, DK and LF10 membranes. With the exception of the NF270

membrane, acetic acid retention was favoured over butyric acid, suggesting that the rejection

of acids during nanofiltration is a complex phenomenon with contributing factors being

concentration, charge and molecular weight.

3.3.2. Nanofiltration of Treated Digested Agricultural Wastewater Using Salts

Solutions

The influence of the addition of four different salts solutions in two different concentrations,

into treated agricultural sludge has been examined. When 50 mM of sodium carbonate and

bicarbonate were added, retention was improved (Fig.8 a, b) for butyric acid (sodium

bicarbonate addition) at a mean of 45.54%, when compared to the untreated feed. Sodium

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bicarbonate did not have a positive effect for all the membranes for acetic acid reducing the

retention by 20%, while sodium carbonate does slightly enhance retention for the HL and DL

membranes, but reduces the retention for DK, NF270 and LF10 by 18% for acetic acid and

12% for butyric acid, possibly due to the dilution effect.

The experimental process was repeated using 100 mM of sodium carbonate and bicarbonate,

with the higher concentration of salts strongly influencing the retention. A different pattern of

results was found (see Fig. 8 a, b) with acetic acid retention being improved by 51% with

both salts, however for butyric acid and the NF270 and LF10 membranes the retention was

reduced by 12.8% and 7.7% respectively. Although the results are encouraging regarding

butyric acid as well as the function of salts as selectivity agents, favoring the retention of one

acid over the other, the retention achieved is not exceptionally high. Further investigation

using sodium chloride and calcium chloride was performed (Fig. 9 a, b). Acetic acid retention

was facilitated although butyric acid was preserved as well, with sodium chloride having a

stronger influence than calcium chloride. In both concentrations, 50 mM and 100 mM sodium

chloride enhanced the retention of acetic acid by 41.2% and 70.7% correspondingly when

compared with the untreated feed, while 50 mM and 100 mM of calcium chloride enhanced

their retention by 62.9% and 41.6% respectively (Fig. 9a). A different trend was found for

butyric acid (Fig.9b) with 50 mM sodium chloride enhancing the retention by 28.4% while

100 mM of sodium chloride reduced the retention by 21.3% when compared to the untreated

samples. This is possibly due to the dilution effect, but also to the selectivity function of the

membranes under sodium chloride addition favoring acetic acid. This effect is reversely

mirrored with calcium chloride were 50 mM of calcium chloride reduced the retention and

100 mM of calcium chloride enhanced the rejection by 32.3% when compared to the

untreated feed.

There is variation in the flux (Table 7) for each membrane in relation to the addition of salts

into the synthetic mixtures, with the highest flux occurring with the addition of calcium

chloride and sodium carbonate, correlating with the minimized retention of the acids from the

membranes. The lowest flux was found with the LF10 membrane and the highest was with

the HL membrane, while the NF270, DL and DK membranes were intermediate.

Regardless of all the nanofiltration membranes being negatively charged polyamide based,

different retention ratios and permeate flux results were achieved under the same operating

conditions (temperature, stirring speed, feed composition, pH, enrichment treatments). These

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differences can be attributed to multiple factors, including the membranes surface

morphology and structure (Vrijenhoek et al., 2001) surface material composition, porosity

and permeability and the composition of the feed solution. It can be summarized that under

the varying settings, the HL membrane had the overall lowest retention rate while the LF10

membrane showed the most promising results. Interestingly, it is found that retention of one

acid is favored versus the other for example acetic acid is better retained than butyric acid and

vice versa. These phenomena are noticeable, with both enrichment treatments in synthetic

mixture solutions and treated wastewater.

During the filtration trials, preference was shown regarding retention among the two acids of

interest, in most cases favouring acetic acid both in standard solutions and treated

wastewater. This proves the complex mechanisms governing nanofiltration, since not only

steric based separations are taking place, but mostly the separation function is based on the

electrochemical interactions between the solutes and the membrane surface.

Results of the nanofiltration tests using the selected membranes were promising regarding the

retention of organic acids with actual wastewater, contrary to the retention results achieved

with synthetic solutions. This might be explained by the complex nature of actual wastewater

since in a multicomponent solution several other factors such as ion content and high organic

content do influence the overall rejection of the acids.

Previously published work (Masse et al., 2008; Weng et al., 2009) has assessed the influence

of operating conditions regarding pressure, during nanofiltration of mixed solutions

containing low molecular weight substances. It has been found that high pressure conditions,

10 bar and above positively influence the retention (Koyuncu, 2002). Therefore, the pressure

applied during the experimental trials summarized in this project was kept at 10 bar, aiming

at an enhanced outcome regarding retention of acids, especially when combined with alkali

conditions.

Since salt mixtures are used in enrichment treatments, osmotic pressure is expected to play an

important role in the separation function of the membranes. The content in total dissolved

solids, alkalinity and conductivity (Table 1) do imply an amount of salts already present in

the treated wastewater. The osmotic pressure has been calculated based on the alkalinity

defined as milligrams of calcium carbonate equal to a molar concentration of 0.02 mol.

Therefore the osmotic pressure exercised by the pre-existing salts has been calculated at

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0.005 kPa. Calculations have also been made regarding the osmotic pressure exercised by

amount of metal ions; the osmotic pressure does not surpass 0.01 kPa. Even with the

addition of numerous salt components, the osmotic pressure remains low, varying between

and not surpassing 0.01(Na2CO3) to 0.03 (NaCl) kPa due to the low concentration of the salts

used. It was found that osmotic pressure does not significantly influence the retention of the

acids.

In polyamide based nanofiltration membranes, the role of the feed solution pH is proven to be

important. The pH has been found influencing the membranes surface charge characteristics

therefore the permeate flux and the retention percentages, contributing to the membranes

separation function stability and efficiency (Ahmad et al., 2008). Alkali conditions have been

found to contribute to changes in the morphology of membranes due to hydration swelling on

the membrane surface layer (Freger et al., 2000), reducing membrane pore size thus reducing

the flux (Ahmad et al., 2008). In alkali conditions, the organic solutes are highly ionized,

which with the simultaneous increase in electronegativity of the membrane surface (Schaep et

al., 1998) enhances retention. In acidic pH ( 7.0) (Table 2). On the other

hand, numerous production processes relevant to the production of platform chemicals such

as acetic and butyric acid, generate mixed inorganic-organic waste streams, often produced

by salt forming reactions or by acid or alkali generating reactions followed by neutralization

(Kertest and King, 1986), hence testing the effect of numerous salts solutions on the retention

of carboxylic acids was of high significance. Free passage of salt would be an advantage if

combined with high retention, nonetheless previous research (Freger et al., 2000) has shown

that increased hardness (Ca+) resulted in decreased retention and membranes with larger

pores are affected more by inorganic ions than tighter membranes (Freger et al., 2000). This

is confirmed as well by the findings of this research, in the case of the HL membrane in

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particular, where the addition of CaCl2 lowers the retention results both in the case of model

solutions and treated wastewater.

The salts were added in a 1:1 volumetric ratio, in model solutions and treated wastewater,

resulting in varying responses regarding retention of acetic and butyric acid of each

membrane. It is apparent that the original concentration of the synthetic mixture of acids is

halved on addition of the salts mixture, due to the effect of dilution. However, whereas in the

case of model solutions, the volumetric ratio and the concentration ratio remain the same, for

treated wastewater the molar concentration ratio is changing significantly when using 50 mM

and 100 mM of salt solutions, favoring the concentration of salts. Therefore, the effects of

salts on the membrane surface are stronger, affecting the membranes surface charge and

enhancing the retention of the acids, in certain cases.

Published research (Bruni and Bandini, 2008) has identified that polymeric based membranes

show diverse behavior regarding flux and retention, depending on the type of electrolytes that

are in contact with them. In that case, retention is mainly governed by the formation of

membrane surface charges, which is activated by the electrolytes type, concentration and

characteristics (Bruni and Bandini, 2008). Normally, single univalent salts such as NaCl or

KCl have a smaller retention percentage when compared with multivalent asymmetric salts

such as Mg2Cl, under constant pH conditions. Simultaneous presence of negatively and

positively charged groups, triggers electrostatic interaction phenomena, known as counter-ion

site-binding (Lyklema, 1995), where ionized surfaces show a tendency to adsorb counter ions

and repel same charge ions. Consequently the addition of salts serves in enhancing the

membranes surface charge in order to repel and so to retain the negatively charged acids.

4. Conclusions

It has been pointed out that farming waste effluents do represent an environmental hazard as

well as a good source of useful nutrients and metals. Developing a complete recovery

strategy for these substances, with a waste treatment system placed in situ could be of great

benefit for the industry. This study investigated spent digester fluids and developed a

recovery strategy solely devoted to the recovery of carboxylic acids from anaerobic

digestates. Membrane filtration was the chosen recovery methodology since it is a suitable

technology for treatment and separation of low molecular substances since these can be

clarified, fractionated, and concentrated to produce high value streams at low cost. Membrane

(19)

technology offers various advantages comparing to its counterparts including scalability,

applicability in a wide range of streams, limited waste generation due to the potential of

recycling. Nanofiltration can be used as a method of isolation and recovery of nutrients from

complex effluent streams, provided a pretreatment scheme is put in place that will remove

coarse particles, so the effluents can be easily filtered. However, the wide adaptation of these

processing schemes is strongly correlated with the cost efficiency of these applications when

compared to the conventional methods of production and recovery of carboxylic acids.

Estimating the cost of these processes though is rather complicated as several factors have to

be taken into consideration, such as capital cost related to manufacturing and maintenance of

the system and relevant equipment, labor costs, energy consumption and transportation of

waste material.

This paper has shown that

During nanofiltration the use of alkali treatment, especially on the digester effluents at

pH 8.5 and 9, enhanced the retention of the CA for all 5 membranes.

Among the membranes tested, the LF10 membrane had the highest retention results

for acetate and butyrate (72.2% and 69.7%, respectively) at pH 8.5, followed by the

NF270 membrane (52.6% and 69.7%) and the DK membrane (57.2% and 45.2%).

The NF270 and DK membranes have a flux rate of 15.40 and 16.49 L/(m2h) at pH 8.5

while the LF10 membrane has a flux rate of 6.40 L/(m2/h), proving this membrane to

be unsuitable for separation of CA at this stage, since operating the system at higher

pressure might be proven uneconomical. The LF10 membrane could be possibly be

used to further concentrate the CAs after they have been successfully separated by

NF270 and DK membranes.

The use of salts, especially of calcium chloride and calcium bi-carbonate in 50 mM

and100 mM concentrations enhanced the retention of the CA for all the 5 membranes.

The addition of salts acts selectively between the acids offering the potential of

recovery of each acid individually, an option that might lead to the fabrication of high

purity acids instead of mixtures.

DL, NF270 and LF10 membranes have an exceptionally good performance with all

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the three different salts added in both concentration ranges

The findings in this paper show potential and could be applied to the biotechnological

production of CA and their recovery.

Acknowledgements

This project was supported by Low Carbon Research Institute (LCRI) project grant title

Wales H2 Cymru. The authors would like to thank Mr. Chris Morris, Technical Director

and Ms. Denise Nicholls, Business Manager, Fre-energy Farm, Wrexham, Wales, United

Kingdom, for providing the team with anaerobically digested agricultural wastewater.

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Table 1: The physical characteristics and chemical composition of the treated digested agricultural wastewater (Gerardo et al., 2013, Zacharof & Lovitt, 2014).

a The collected samples were diluted 100 times with deionised water and measured in a 1 cm light path

Parameters

Treated digested agricultural wastewater

Microfiltered (0.2 m) Permeate

Total Solids (TS, g/L) 6.04

Total Suspended Solids (TSS, mg/L) 190

Total Dissolved Solids (TDS, mg/L) 4250

pH 8.25

Conductivity (mS/cm) 5.30

Alkalinity (mg CaCO3/L) 2287

Zeta Potential (mV) -24.20

Sizing (m) 2.93

Optical Density (580nma)

0.10

Concentration mg/L mM/L

Acetic Acid 1265.85 21.08

Butyric Acid 1393.02 15.81

Metal ions (Ca,Cu, Co,Fe, Pb, Mg, Mn, Zn,K As) 880.00 22.65

Ammonia 686.19 40.29

Phosphate 41.51 0.43

(2)

Table 2: Membranes characteristics provided by the manufacturers and in the literature (Choi et al. 2008;Al-Amoudi et al. 2007, 2008)

Characteristics Membranes

Manufacturer General Electric -Osmonics USA Dow FilmTech USA Nitto Denko Japan

Model HL DL DK NF 270 LF10

Distributors Sterlitech Corporation

http://www.sterlitech.com

Desal Supplies

http://www.desal.co.uk

SOMICON AG WKL

http://www.somicon.com

Material Thin film composite piperazine

based polyamide microporous

polysulfone

Thin film composite-aromatic

polyamide

Thin film composite Polyvinyl

alcohol-aromatic cross linked

polyamides

Applications Water Softening, Acid Purification, Detergent removal, Heavy metal removal

Geometry Flat Sheet

Effective Membrane area (cm2) 14.60

Flux rate [L/(m

2 h)] at 689 kPa 66.3 52.7 37.4 122.0 11.9

Charge (at neutral pH) Negative

pH 2-10 2-11 3-10 2-10

Ion rejection (%) 98 96 98 97 99.5

MWCO 150-300 150-200

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Table 3: The influence of pH and membrane type on permeate flux of deionised water, using a variety of nanofiltration membranes at 10 bar operating

pressure.

Deionised Water Permeate Flux [L/(m

2 h)]

No adjustment Adjusted pH

pH 7.2 4.0 5.5 7.0 8.5 9.0

Membranes DK 44.60 53.08 46.15 44.91 44.47 38.07

DL 56.02 65.20 58.35 57.66 56.87 49.65

HL 118.43 148.03 130.94 120.12 117.35 100.66

NF270 27.40 33.08 29.56 29.29 23.63 17.43

LF10 15.95 26.53 19.29 14.11 14.09 12.51

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Table 4: The effect of pH on permeate flux of synthetic solutions of known acid concentrations (50 mM, 100 mM acetic/butyric acid) using a variety of

nanofiltration membranes.

Permeate Flux [L/(m2 h)]

Solution 50 mM Synthetic Solution of Acetic-Butyric acid

Permeate

100 mM Synthetic Solution of Acetic-Butyric acid

Permeate

pH 4.0 5.5 7.0 8.5 9.0 4.0 5.5 7.0 8.5 9.0

Membranes DK 23.91 19.43 15.44 14.94 7.47 28.80 23.40 18.60 18.00 9.00

DL 32.86 19.42 11.95 11.45 10.95 39.60 23.40 14.40 13.80 13.20

HL 43.82 34.86 34.36 26.89 21.42 52.80 42.00 41.40 32.40 25.80

NF270 22.91 20.42 14.44 19.46 13.98 27.60 24.60 17.40 14.80 11.40

LF10 16.43 12.95 7.97 6.97 4.98 19.80 15.60 9.60 8.40 8.14

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Table 5: The effect of calcium chloride, sodium chloride and sodium carbonate and bicarbonate solutions on permeate flux of synthetic solutions of known acid

concentrations (50 mM, 100 mM acetic/butyric acid) using a variety of nanofiltration membranes.

Permeate Flux [L/(m2 h)]

Solution 50mM Synthetic Solution of Acetic-Butyric acid Permeate 100 mM Synthetic Solution of Acetic-Butyric acid Permeate

pH 3.88 3.14 10.72 10.52 3.67 2.97 10.42 10.21

Salts solutions 50 mM

CaCl

50mM

NaCl

50 mM

Na2CO3

50 mM

NaHCO3

100 mM

CaCl

100mM

NaCl

100 mM

Na2CO3

100 mM

NaHCO3

Membranes DK 22.80 25.68 22.49 28.20 14.99 21.91 25.48 24.24

DL 33.00 33.09 30.38 27.23 19.80 24.08 33.66 28.36

HL 37.76 42.00 30.68 38.24 22.16 31.53 37.67 44.04

NF270 23.17 28.20 29.57 26.78 11.78 17.05 20.03 37.50

LF10 18.24 21.11 14.80 15.25 17.10 15.53 12.41 17.30

(6)

Table 6: The effect of pH on permeate flux of standardised anaerobically digested fluids using a variety of nanofiltration membranes. The filtration fluids were

derived from microfiltered sludge (see Table 1)

Permeate Flux [L/(m2 h)]

Solution No adjustment MF (0.2 m) Sludge Permeate Adjusted pH

Membranes 8.25 4.0 5.5 7.0 8.5 9.0

DK 16.00 21.48 21.42 17.64 16.49 2.09

DL 14.47 18.33 17.92 16.78 14.91 5.06

HL 14.95 25.48 22.55 20.04 14.37 11.42

NF270 16.04 21.70 20.75 19.05 15.40 3.04

LF10 6.58 13.35 12.09 6.00 5.44 4.14

(7)

Table 7: The effect of calcium chloride, sodium chloride and sodium carbonate and bicarbonate solutions on permeate flux of synthetic solutions of

standardised anaerobically digested fluid acid (see Table 1) using a variety of nanofiltration membranes.

Permeate Flux [L/(m2 h)]

Solution MF (0.2 m) Sludge Permeate

pH 8.25 8.13 8.97 9.92 9.62 7.98 8.55 9.87 9.37

Salts solutions No treatment 50 mM

CaCl

50mM

NaCl

50 mM

Na2CO3

50 mM

NaHCO3

100 mM

CaCl

100mM

NaCl

100 mM

Na2CO3

100 mM

NaHCO3

Membranes DK 16.00 17.86 16.28 11.40 14.52 14.99 16.86 10.49 8.55

DL 14.47 15.60 18.48 24.88 22.78 19.80 17.05 19.70 11.70

HL 14.95 22.16 21.56 19.18 23.01 28.06 22.20 18.65 16.65

NF270 16.04 20.14 22.17 10.00 11.78 14.10 12.60 14.70 12.70

LF10 6.58 16.86 13.68 8.55 7.50 7.30 7.52 3.60 5.92

(8)

Figure 1:Schematic representation of the high pressure stirred cell unit [1] nitrogen cylinder, [2] pressure regulator valve, [3] pressure indicator , [4] stirred cell unit equipped with

membrane disc [5] stirrer, [6] stirring plate, [7] permeate collection vessel, [8] high precision electronic scale, [9] personal computer, [10]membrane disc.

(9)

Figure 2: Processing and recovery scheme for carboxylic acids

(10)

Figure 3 [a, b]:The effect of pH on carboxylic acid retention (a) acetic acid (b) butyric acid of a variety of NF membranes using synthetic solutions (50 mM acetic/butyric acid).

-40

-20

0

20

40

60

80

100

3 5 7 9 11

Ret

en

tio

n (

%)

pH

DK

DL

HL

NF270

LF10

(a)

-40

-20

0

20

40

60

80

100

3 5 7 9 11

Ret

en

tio

n (

%)

pH

DK

DL

HL

NF270

LF10

(b)

(11)

Figure 4 [a, b]:The effect of pH on carboxylic acids retention (a) acetic acid (b) butyric acid of a variety of NF membranes using synthetic solutions (100 mM acetic/butyric

acid).

-40

-20

0

20

40

60

80

100

3 5 7 9 11

Ret

en

tio

n (

%)

pH

DK

DL

HL

NF270

LF10

(a)

-40

-20

0

20

40

60

80

100

3 5 7 9 11

Ret

en

tio

n (

%)

pH

DK

DL

HL

NF270

LF10

(b)

(12)

Figure 5 [a, b]: The effect of sodium carbonate (Na2CO3) and sodium bicarbonate(NaHCO3) solutions on carboxylic acids retention (a) acetic acid (b) butyric acid of a variety of NF

membranes using synthetic solutions of known acid concentrations (50 mM, 100 mM acetic/butyric acid).

0

20

40

60

80

100

Without any treatment50mM Acetic acid

50mM Acetic acid-50mMNa2CO3

50mM Acetic acid-50mMNaHCO3

Without any treatment100mM Acetic acid

100mM Acetic acid -100mM Na2CO3

100mM Acetic acid -100mMNaHCO3

Ret

en

tio

n (

%)

Treatments

(a) DKDLHLNF 270

Butyriu

0

20

40

60

80

100

Without any treatment50mM Butyric acid

50mM Butyric acid-50mMNa2CO3

50mM Butyric acid-50mMNaHCO3

Without any treatment100mM Butyric acid

100mM Butyric acid-100mM Na2CO3

100mM Butyric acid-100mM NaHCO3

Ret

en

tio

n (

%)

Treatments

(b) DK

DL

HL

NF 270

Butyriu

(13)

Figure 6 [a, b]:The effect of sodium chloride (NaCl) and calcium chloride (CaCl) solutions on carboxylic acids retention (a) acetic acid (b) butyric acid of a variety of NF

membranes using synthetic solutions of known acid concentrations (50 mM, 100 mM acetic/butyric acid).

0

10

20

30

40

50

60

70

80

90

100

Without any treatment50mM Acetic acid

50mM Acetic acid -50mMNaCl

50mM Acetic acid -50mMCaCl

Without any treatment100mM Acetic acid

100mM Acetic acid -100mM NaCl

100mM Acetic acid -100mM CaCl

Ret

en

tio

n (

%)

Treatments

(a) DK

DL

HL

NF 270

Butyriu

0

20

40

60

80

100

Without any treatment50mM Butyric acid

50mM Butyric acid-50mMNaCl

50mM Butyric acid-50mMCaCl

Without any treatment100mM Butyric acid

100mM Butyric acid-100mM NaCl

100mM Butyric acid-100mM CaCl

Ret

en

tio

n (

%)

Treatments

(b) DK

DL

HL

NF 270

Butyriu

(14)

Figure 7 [a, b]:The effect of pH on carboxylic acids retention (a) acetic acid (b) butyric acid of a variety of NF membranes using standardised anaerobically digested fluids. The

filtered fluids are permeates derived from microfiltration of agricultural sludge (see Table 1)

-40

-20

0

20

40

60

80

100

3 5 7 9 11

Ret

en

tio

n (

%)

pH

DK

DL

HL

NF270

LF10

(a)

-40

-20

0

20

40

60

80

100

3 5 7 9 11R

ete

nti

on

(%

) pH

DK

DL

HL

NF270

LF10

(b)

(15)

Figure 8 [a, b]:The effect of sodium carbonate (NaCO3) and sodium bicarbonate (NaHCO3) solutions carboxylic acids retention (a) acetic acid (b) butyric acid of a variety of NF membranes

using standardised anaerobically digested fluids. The filtered fluids are permeates derived from microfiltration of agricultural sludge (see Table 1)

0

20

40

60

80

100

Without any treatment MF Permeate -50mM Na2CO3 MF Permeate -50mM NaHCO3 MF Permeate -100mM Na2CO3 MF Permeate -100mM NaHCO3

Ret

enti

on

(%

)

Treatments

(a) DKDLHLNF 270LF10

Butyriu

0

20

40

60

80

100

Without any treatment MF Permeate -50mM Na2CO3 MF Permeate -50mM NaHCO3 MF Permeate -100mM Na2CO3 MF Permeate -100mM NaHCO3

Ret

enti

on

(%

)

Treatments

(b) DK

DL

HL

NF 270

Butyriu

(16)

Figure 9 [a, b]:The effect of sodium chloride (NaCl) and calcium chloride (CaCl) solutions carboxylic acids retention (a) acetic acid (b) butyric acid of a variety of NF membranes using

standardised anaerobically digested fluids. The filtered fluids are permeates derived from microfiltration of agricultural sludge (see Table 1)

0

10

20

30

40

50

60

70

80

90

100

Without any treatment MF Permeate -50mM NaCl MF Permeate -50mM CaCl MF Permeate -100mM NaCl MF Permeate -100mM CaCl

Ret

en

tio

n (

%)

Treatments

(a) DKDL

HL

NF 270

Butyriu

0

20

40

60

80

100

Without any treatment MF Permeate -50mM NaCl MF Permeate -50mM CaCl MF Permeate -100mM NaCl MF Permeate -100mM CaCl

Ret

en

tio

n (

%)

Treatments

(b) DKDLHLNF 270

Butyriu